Brush seals provide an effective seal against fluid media, in which the tightly packed bristles form a flexible contact, generally with a rotating member. The projecting, free ends of the bristles must be cut evenly and smooth, to provide both enhanced sealing and reduced wear on the surface of the rotating member. Three basic procedures have been employed to achieve such cutting.
One such method, conventional bore grinding of the bristles, is shown in U.S. Pat. No. 4,678,113. In this process, probably the slowest of the three basic methods, a power driven grinding wheel is mounted for axial movement relative to a supported seal member. The seal is rotated while at the same time the grinding wheel is rotated and reciprocated axially to grind the internal bore of the seal. This method is not widely employed since it has been found difficult to maintain a true bore. Additionally, with respect to metal bristles, bore grinding is the slowest of the three known techniques.
The '113 patent also notes that the inner diameter may be finished by tip grinding. The tip grinding process is shown in greater detail in U.S. Pat. No. 4,839,997, the disclosure of which is incorporated herein, by reference. An abrasive tip cutter is used to cut through the bristles in an axial direction and offers the advantage of being the fastest of the three known techniques. However, this method, analogous to conventional hole saw cutting, requires a different diameter hole saw for each different brush seal diameter. A further disadvantage of this type of cutter is the high cost, due to its short useful life. New cutters must be purchased periodically because of rapid wear, which is soon reflected on the finished bore size. The cutter design, itself, is primarily responsible for this wear. Rather than using saw teeth as in the conventional hole saw cutter, cutting is achieved by attaching abrasive grit to the sharp edge of the cutter. Grit adhesion strength on the sharp edge is low because of the small amount of contact between the grit particle and the fine edge--resulting in grit particles breaking free from the substrate edge.
The other method for machining bristles is Electro Discharge Machining, the disadvantages of which are also shown in the '997 patent. A disadvantage of this procedure is its high cost requiring the purchase of an expensive EDM machine--with the expense increasing dramatically for large diameter brush seals. The cutting process is slow, and there is generally a recast layer left by the cutting process, which must subsequently be removed by an abrasive technique--further adding to the cost.